1. Field of the Invention
The present invention generally relates to computer user interfaces of the type that control the position of a cursor on a display screen and, more particularly, to a reduced cost circuit for generating digital signals from the analog strain gauge signals of a pointing stick used to control the cursor position and select objects on the display screen.
2. Background Description
A pointing stick is used, for example, to implement the TrackPoint.TM. cursor controller in International Business Machines' ThinkPad.TM. lap top computers. This pointing stick is constructed with a cantilever beam on which four strain gauges are mounted to detect flexure of the beam in each of three axes. The analog signals from the strain gauges are converted to digital X and Y signals by an analog-to-digital (A/D) converter. These signals are further processed to generate the final control signals which are used to control the position of the cursor on the display screen.
This implementation of the pointing stick has certain disadvantages. First of all, the low-order bits of the voltage values are read by a hardware analog-to-digital (A/D) converter, which is expensive. Secondly, the configuration of the resistances and operational amplifiers required for three-axis capability interact in a way that requires that at least one of the operational amplifiers be of "instrument" quality, roughly tripling or quadrupling its cost. In addition, the operational amplifiers need to be linear and have a wide output range (rail to rail), which increases their cost.
Consider a strain gauge sensor configuration of two approximately balanced gauges on opposite sides of a cantilever beam, wired in series across a voltage source. The voltage at the junction of the two gauges gives a measure (relative to the supply voltage) of the relative resistances of the two gauges, and thus of the flexure of the beam in the direction connecting the gauges. If an additional fixed resistance is connected in series with the gauges, then the voltage across this resistance gives a measure of the total resistance of the gauges, and thus of the compression/extension of the beam. The changes are to be measured with a precision of a few parts in 10.sup.5, in the presence of variations in the source voltage of a few percent. A dynamic range of about 250 times the precision is required, but the location of this range is predictable at manufacturing only to within a few percent. Cost is a major consideration; a difference of $1 may decide competitive success.